In this article we will discuss about:- 1. Introduction to Uredinales 2. Nuclear Cycle of Uridinales 3. Biological Specialization 4. Classification.
Introduction to Uredinales:
The Uredinales, or rust fungi comprise a large group of obligate parasites. They are called rusts, because of the rusty, reddish-brown colour of their spores, which are formed chiefly upon surfaces of host leaves and stems in pustular outgrowths (Fig. 261 A, D & E).
Individual species of rusts may have a very wide or a very limited range of hosts. Parasitism and specialization are so highly developed among them that thus far, except a few cases almost all attempts to grow them on artificial media have failed. Rusts are parasitic on seed plants or ferns. They cause great loss to crop, forest, and ornamental plants.
No other group of fungi is so injurious to plants useful to man. They are considered as one of the virulent plant parasites.
Some rust fungi complete their life cycle on a single kind of host, they are known as autoecious rusts. But others require two unrelated hosts for the completion of life cycle, they are called heteroecious rusts. Most autoecious rusts are of relatively little economic importance, but the heteroecious ones cause some of the most destructive of all plant diseases.
Among the important heteroecious rusts are: white pine blister rust (Fig. 261G), Cronartium ribicola, which has currents and gooseberries (the genus Ribes), especially Ribes nigrum, as alternate hosts with white pine (Pinus strobus); and black stem rust of wheat, Puccinia graminis tritici has wheat (Triticum aestivum) and barberry (Berberis vulgaris) as alternate hosts.
Among the more common autoecious rusts are: hollyhock rust, Puccinia malvacearum on species of Althaea, Levatera, Malva, all genera of the Malvaceae; and flax rust, Melampsora lini on species of Linum.
Rust fungi do not produce fruiting bodies. Life cycle of most of them is extremely complicated. It usually involves as many as five kinds of spore forms which appear in a definite succession. The life cycle is thus polymorphic.
The five spore forms are:
Pycniospores (spermatia), aeciospores, uredospore’s, teleutospores, and sporidia (basidiospores).
These spore forms are also expressed as follows:
Stage 0 Spermogonial stage producing spermogonia bearing spermatia and receptive hyphae (n).
Stage I Aecial stage producing aecium bearing aeciospores (n+n).
Stage II Uredial stage producing uredia bearing uredospores (n+n).
Stage III Telial stage producing telia bearing teleutospores (n+n), 2n.
Stage IV Sporidial stage producing promycelium bearing sporidia (n).
The rust fungus producing either all the five spore forms or at least one type of dikaryotic spore in addition to the teleutospore, is referred to as a long-cycled or macrocyclic rust.
Macrocyclic rusts again may be heteroecious (e.g., Puccinia graminis), which produces all spore forms on two unrelated hosts; or autoecious (e.g., Puccinia asparagi) whose all spore forms are produced on asparagus. The prefix ‘eu’ is sometimes attached to the generic name as a descriptive epithet, i.e., Puccinia graminis is an eu-Puccinia.
Whereas in a short-cycled or microcyclic rust, the teleutospores are the only dikaryotic spores produced in the life cycle. The life cycle of rusts is said to be demicyclic in which uredospores are lacking; as in heteroecious forms in Cymnosporangium, and amongst autoecious forms in Xenodochus carbonarius.
It is believed that the heteroecious macrocyclic forms represent the primitive condition, and that the autoecious conditions arose later in evolution. It is also believed that forms with shorter life cycles arose during the course of evolution from macrocyclic ancestor.
The mycelium of rust fungi usually ramifies intercellularly in the host tissue sending haustoria in the adjacent cells and inducing malformations in the infected parts (Fig. 261B). The infection may be localized or systemic. The mycelium may be short-lived or perennial and consisting of well-marked phases that alternate: a monokaryotic phase and a dikaryotic phase.
Clamp connections have been found in the dikaryotic mycelium of very few rusts.
The monokaryotic mycelium develops:
Stage 0 (Spermogonial stage) and the dikaryotic mycelium whose activity is rather more predominating than the monokaryotic mycelium produces
Stage I (Aecial stage),
Stage II (Uredial stage), and
Stage III (Telial stage).
In the late telial stage karyogamy takes place. The Stage IV (Sporidial stage) is the one where meiosis takes place and mono: karyotic condition is established. In the Uredinales the sporidia fall into two compatible groups and are determined genetically by a pair of allelomorphs (A, a).
The dikaryotic condition is initiated in the late Stage 0. Craigie (1927) working on Puccinia graminis showed that the dikaryotic condition in rusts is achieved by the fusion of spermatia with receptive hyphae of opposite strains. Brown in 1932 found that in Puccinia helianthi, an autoecious rust, a dikaryotic mvcelium of uredosorus can induce aecial formation in a monokaryotic mycelium produced from a sporidial infection.
Such a process of diploidization of a monokaryotic mycelium by a dikaryotic mycelium is known as the Buller phenomenon. Later on Brown (1935) found evidence that in P. helianthi, the dikaryotic condition is also established by the fusion of monokaryotic hyphae of spermogonia of two opposite strains. In many rust fungi, only the aecial stage alone is known.
Again in many others, the uredial stage has been worked out. It has become necessary at present to name and classify the uredial, aecial and other non-telial forms of which the telial stage is not yet known. For convenience of reference they are classified under an artificial group ‘Uredinales Imperfecti’ in which are included the form-genera Caeoma, Aecidium, Petidermium, Roestelia, and Uredo.
Nuclear Cycle of Uredinales:
In all the rust fungi the dikaryotic condition (n +n) is rather more well represented than the monokaryotic stage (n). It is established by spermatization between a (+) spermatium and a (—) receptive hypha and vice versa. Immediately after dikaryotization, the dikaryotic hyphal connection that already existed between the teleutospore of Puccinia graminis spermogonium and protoaecium.
The protoaecium which showing promycelium and has become dikaryotic, develops into an aecium bearing sporidia. B. Telial column of dikaryotic hyphal cells. The aeciospores borne in the Cronartium sp. showing germinaaecium are also dikaryotic. The dikaryotic condition’s ting teleutospores.
Further continued through dikaryotic mycelia that are formed from the aeciospores. The dikaryotic mycelia induce infection in the host and produce uredia and urediospores which are also dikaryotic. The dikaryotic condition is further perpetuated by the development of telia and teleutospores. End of the dikaryotic condition comes with the development of teleutospores.
The teleutospores when young are dikaryotic and at maturity karyogamy takes place when they become diplophasic (2n). Meiosis of the diploid nucleus takes place during the teleutospore germination when the monokaryotic condition (n) is established.
The monokaryotic condition is represented by sporidia and continues up to the development of spermogonia, spermatia, receptive hyphae and finally the protoaecia with which the life cycle is completed.
Of the stages 0 to IV, only the stages 0 and IV are monokaryotic while the rest of the stages I to III of the life cycle are dikaryotic with a very short diplophase. All rust fungi perennate in the dikaryotic condition either as mycelium or as uredospores or as teleut Dspores. But the teleutospores serving as perennating organs may also be in the diplophasic condition.
Besides this, it is during the dikaryotic condition when maximum damage is caused to the host.
Biological Specialization of Uredinales:
Specialization of parasitism is one of the outstanding characteristics of the rust fungi. Certain rusts may be confined to a single host species even variety, while others are capable of infecting many different hosts which may be related or unrelated. Again there are others, each of which is found on a large number of hosts.
Upon its numerous hosts the fungus may show no morphological variation, yet attempts to inoculate from one host to another may uniformly produce negative results.
Such a biological specialization of parasitism has necessitated to introduce the term biological species. Hence a biological species be regarded as a collection of morphologically similar organisms or strains that differ in ability to produce infection in different hosts. The existence of biological species has been amply demonstrated among cereal rusts for example, Puccinia graminis.
Classification of Uredinales:
The Uredinales are classified into three families on the basis of their teleutospore characteristics and behaviour.
The three families are: Coleosporiaceae, Melampsoraceae, and Pucciniaceae.
Key to the families is presented below:
A . Teleutospore germination by the formation of an external promycelium
B. Teleutospores usually stalked, less commonly sessile, free or variously united but never form layers or crust Pucciniaceae
BB. Teleutospores sessile laterally united forming crusts or cushions or columnar masses
AA. Teleutospore germination by the formation of an internal promycelium